package suanfa;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class SumOfFourNumbers {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> ret=new ArrayList<>();
        Arrays.sort(nums);
        int n=nums.length;
        for(int i=0;i<n;){
            for(int j=i+1;j<n;){
                int left=j+1,right=n-1;
                long aim=(long) target-nums[i]-nums[j];
                while(left<right){
                    int sum= nums[left]+nums[right];
                    if(sum>aim) right--;
                    else if(sum<aim) left++;
                    else{
                        ret.add(Arrays.asList(nums[i],nums[j],nums[left],nums[right]));
                        left++;
                        right--;
                        while(left<right && nums[left]==nums[left-1]) left++;
                        while(left<right && nums[right]==nums[right+1]) right--;
                    }
                }
                j++;
                while(j<n && nums[j]==nums[j-1]) j++;
            }
            i++;
            while(i<n && nums[i]==nums[i-1]) i++;
        }
        return ret;
    }
}
